Ultrahard diamonds and method of making thereof

ABSTRACT

A single crystal diamond grown by microwave plasma chemical vapor deposition annealed at pressures in excess of 4.0 GPa and heated to temperature in excess of 1500 degrees C. that has a hardness of greater than 120 GPa. A method for manufacture a hard single crystal diamond includes growing a single crystal diamond and annealing the single crystal diamond at pressures in excess of 4.0 GPa and a temperature in excess of 1500 degrees C. to have a hardness in excess of 120 GPa.

This application is a continuation of U.S. patent application Ser. No.11/401,288, filed on Apr. 11, 2006 now U.S. Pat. No. 7,309,477 which isa divisional application of application Ser. No. 10/889,170, filed onJul. 13, 2004 now U.S. Pat. No. 7,115,241 and claims the benefit ofProvisional Application Ser. No. 60/486,435 filed on Jul. 14, 2003,which is hereby incorporated by reference.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with U.S. government support under grant numberEAR-0135626 from the National Science Foundation. The U.S. governmenthas certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to diamonds, and more particularly, to anultrahard diamond produced using a Microwave Plasma Chemical VaporDeposition (MPCVD) within a deposition chamber.

2. Description of Related Art

Large-scale production of synthetic diamond has long been an objectiveof both research and industry. Diamond, in addition to its gemproperties, is the hardest known material, has the highest known thermalconductivity, and is transparent to a wide variety of electromagneticradiation. Thus, diamond is valuable because of its wide range ofapplications in a number of industries, in addition to its value as agemstone.

For at least the last twenty years, a process of producing smallquantities of diamond by chemical vapor deposition (CVD) has beenavailable. As reported by B. V. Spitsyn et al. in “Vapor Growth ofDiamond on Diamond and Other Surfaces,” Journal of Crystal Growth, vol.52, pp. 219-226, the process involves CVD of diamond on a substrate byusing a combination of methane, or another simple hydrocarbon gas, andhydrogen gas at reduced pressures and temperatures of 800-1200° C. Theinclusion of hydrogen gas prevents the formation of graphite as thediamond nucleates and grows. Growth rates of up to 1 μm/hour have beenreported with this technique.

Subsequent work, for example, that of Kamo et al. as reported in“Diamond Synthesis from Gas Phase in Microwave Plasma,” Journal ofCrystal Growth, vol. 62, pp. 642-644, demonstrated the use of MicrowavePlasma Chemical Vapor Deposition (MPCVD) to produce diamond at pressuresof 1-8 Kpa in temperatures of 800-1000° C. with microwave power of300-700 W at a frequency of 2.45 GHz. A concentration of 1-3% methanegas was used in the process of Kamo et al. Maximum growth rates of 3μm/hour have been reported using this MPCVD process.

Natural diamonds have a hardness between 80-120 GPa. Most grown ormanufactured diamonds, regardless of the process, have a hardness ofless than 110 GPa. Other than type IIa natural diamonds, which have beenannealed, diamonds have not been reported to have a hardness of greaterthan 120 GPa.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a an apparatus and amethod for producing diamond that substantially obviates one or more ofthe problems due to limitations and disadvantages of the related art.

An object of the present invention relates to an apparatus and methodfor producing diamond in a microwave plasma chemical vapor depositionsystem having increased hardness.

Another object of the present invention is to enhance the opticalcharacteristics of a single crystal diamond.

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, a singlecrystal diamond grown by microwave plasma chemical vapor depositionannealed at pressures in excess of 4.0 GPa and heated to temperature inexcess of 1500 degrees C. that has a hardness of greater than 120 GPa.

In another embodiment, A single crystal diamond having a hardness of160-180 GPa

In accordance with another embodiment of the present invention, A methodfor manufacture a hard single crystal diamond includes growing a singlecrystal diamond and annealing the single crystal diamond at pressures inexcess of 4.0 GPa and a temperature in excess of 1500 degrees C. to havea hardness in excess of 120 GPa.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

FIG. 1 is a diagram of an indenter for testing the hardness of adiamond.

FIG. 2 is a picture of an indentation made on a diamond.

FIG. 3 is a graph showing the hardness and toughness of annealedmicrowave plasma CVD-grown single-crystal diamonds in comparison to typeIIa natural diamonds annealed type IIa natural diamonds, annealed typeIa natural diamonds and annealed type Ib HPHT synthetic diamonds.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of thepresent invention, the results of which are illustrated in theaccompanying drawings.

The microwave plasma CVD-grown single-crystal diamond referred to inthis application were grown with the apparatus described in U.S. patentapplication Ser. No. 10/288,499 filed on Nov. 6, 2002 entitled“Apparatus and Method for Diamond Production,” which is herebyincorporated by reference. In general, a seed diamond is placed inholder that moves the seed diamond/grown diamond as the diamond isgrown. The inventors of this application are also inventors in U.S.patent application Ser. No. 10/288,499.

A microwave plasma CVD-grown single-crystal diamond having a thicknessof greater than 1 millimeter was deposited on type Ib {100} syntheticdiamond. In order to enhance the growth rate (50-150 μm/h) and promotesmooth {100} face growth, single-crystal diamonds were grown in anatmosphere of N₂/CH₄=0.2-5.0%, CH₄/H₂=12-20%, 120-220 torr totalpressure, and 900-1500° C. from a microwave induced plasma within a CVDchamber. Raman spectra show a small amount of hydrogenated amorphouscarbon (a-C:H)⁴ and nitrogen-containing a-C:H (N:a-C:H)⁴ giving rise tobrown diamond at <950° C. and > 1400° C. Photoluminescence (PL) spectraindicate nitrogen-vacancy (N-V) impurities. Single crystal diamonds upto 4.5 mm in thickness have been fabricated at growth rates that are asmuch as two orders of magnitude higher than conventional polycrystallineCVD growth methods.

The microwave plasma CVD-grown single-crystal diamonds were annealed atpressures in excess of 4.0 GPa, such as 5-7 GPa, and heated totemperature in excess of 1500 degrees C., such as 1800-2900 degrees C.,for 1-60 min in a reaction vessel using a belt-type or anvil-typeapparatus. The reaction vessel can be a cell, such as that described inU.S. Pat. No. 3,745,623 or U.S. Pat. No. 3,913,280, which are herebyincorporated by reference. Such an annealing treatment, reduces oreliminates the color in the microwave plasma CVD-grown single-crystaldiamond crystals, and lightening the tint of the type Ib HPHT syntheticseed crystals. Further, the hardness of the annealed microwave plasmaCVD-grown single-crystal diamond annealed CVD diamond (at least ˜140GPa) is beyond that of annealed or unannealed type Ib HPHT syntheticdiamond (˜90 GPa), annealed type Ia natural diamond (˜100 GPa), type IIanatural diamond (˜110 GPa), and annealed type IIa natural diamond (˜140GPa) and sintered polycrystalline diamond (120-140 GPa).

EXAMPLE #1

A single crystal CVD diamond was grown with a N₂/CH₄ ratio of 5% at atemperature of approximately 1500 degrees C. on a yellow type Ib HPHTsynthetic diamond in a microwave CVD chamber. The dimension of themicrowave plasma CVD-grown single-crystal diamond was one centimetersquare and a little larger than one millimeter in thickness. The colorof the microwave plasma CVD-grown single-crystal diamond was brown. Thebrown microwave plasma CVD-grown single-crystal diamond on the type IbHPHT synthetic seed diamond was then placed as a sample in the reactionvessel.

The reaction vessel was placed in a conventional HPHT apparatus. First,the pressure was increased to a pressure of 5.0 GPa, and then thetemperature was brought up to 2200 degrees C. The sample was maintainedat these annealing conditions for five minutes, and then the temperaturewas decreased over a period of about one minute to room temperaturebefore the pressure was released.

The sample was removed from the reaction vessel and examined under anoptical microscope. The brown microwave plasma CVD-grown single-crystaldiamond had turned to a light green translucent color and remainedfirmly bonded to the yellow type Ib HPHT synthetic diamond. The yellowcolor of the type Ib HPHT synthetic diamond became a lighter yellow or amore translucent yellow. The hardness was about 160 GPa.

EXAMPLE #2

Same as example #1 above, except the annealing conditions weremaintained for 1 hour. The brown microwave plasma CVD-grownsingle-crystal diamond turned to a light green color, which was moretranslucent than the light green color resulting in example #1, andremained firmly bonded to the type Ib HPHT synthetic diamond. The yellowcolor of the type Ib HPHT synthetic diamond became a lighter yellow or amore translucent yellow. The hardness was about 180 GPa.

EXAMPLE #3

A single crystal CVD diamond was grown with a N₂/CH₄ ratio of 5% at atemperature of approximately 1450 degrees C. on a yellow type Ib HPHTsynthetic diamond in a microwave CVD chamber. The dimension of themicrowave plasma CVD-grown single-crystal diamond was one centimetersquare and a little larger than one millimeter in thick. The color ofthe microwave plasma CVD-grown single-crystal diamond was a light brownor yellow. In other words, a yellow or light brown that was not as darkas the brown of the microwave plasma CVD-grown single-crystal diamond inexample #1 above. The yellow or light brown microwave plasma CVD-grownsingle-crystal diamond on the type Ib HPHT synthetic diamond was thenplaced as a sample in a reaction vessel. The hardness was greater than160 GPa.

The reaction vessel was placed in a conventional HPHT apparatus. Thepressure was increased to about to a pressure of 5.0 GPa, and then thetemperature was rapidly brought up to about 2000 degrees C. The samplewas maintained at these annealing conditions for five minutes, and thenthe temperature was decreased over a period of about one minute to roomtemperature before the pressure was released.

The sample was removed from the reaction vessel and examined under anoptical microscope. The light brown microwave plasma CVD-grownsingle-crystal diamond had become colorless and remained firmly bondedto the yellow type Ib HPHT synthetic diamond. The yellow color of thetype Ib HPHT synthetic diamond also became a lighter yellow or a moretranslucent yellow.

EXAMPLE #4

Same as example #1 except a colorless microwave plasma single-crystalCVD-grown diamond in an atmosphere of N₂/CH₄=5% at a temperature of˜1200 degree C. was annealed. After annealing, the microwave plasmasingle-crystal CVD-grown diamond was blue. This blue microwave plasmasingle-crystal CVD-grown diamond had a very high toughness of >20 MPam^(1/2). The hardness was about ˜140 GPa.

EXAMPLE #5

Same as example #1 except a colorless microwave plasma single-crystalCVD-grown diamond in an atmosphere of N₂/CH₄=0.5% at a temperature of˜1200 degree C. was annealed. The microwave plasma single-crystalCVD-grown diamond was still colorless. This colorless microwave plasmasingle-crystal CVD-grown diamond had a hardness of ˜160 GPa andtoughness of ˜10 MPa m^(1/2).

FIG. 1 is a diagram of an indenter for testing the hardness of adiamond. A Vickers hardness test were performed on the annealedmicrowave plasma CVD-grown single-crystal diamonds with the indenter 1shown in FIG. 1. The indenter 1 in FIG. 1 has an impinging material 2positioned on a mount 3. The impinging material 2 can be siliconcarbide, diamond or some other hard material. The impinging material hasa face with a pyramidal Vickers indenter shape in which the sides of thepyramidal Vickers indenter shape have an angle of 136°.

The indenter applies a point load to the test diamond 2 until anindentation or crack is formed in the test diamond 2. To prevent elasticdeformation of the indenter, the loads were varied from 1 to 3 kg on{100} faces in the <100> direction of the test diamonds. Dimensions ofthe indentation and the cracks associate with the indentation aremeasured via optical microscopy. FIG. 2 is a picture of an indentationmade on a microwave plasma CVD-grown single-crystal diamond.

By measuring the length D and height h of the indentation, the hardnessH_(v) of the test diamond can be determined from the following equation(1):H _(v)=1.854×P/D ²  (1)P is the maximum load used on the indenter to form an indentation intothe test diamond. D is the length of the longest crack formed by theindenter in the test diamond and h is the depth of the indentation intothe test diamond, as shown in FIG. 1.

The fracture toughness K_(c) of the test diamond can be determined byusing the hardness H, from equation (1) in the following equation (2):K _(c)=(0.016±0.004)(E/H _(v))^(1/2)(P/C ^(3/2))  (2)E is the Young's modulus, which is assumed to be 1000 GPa. P is themaximum load used on the indenter to form the indentation into the testdiamond. The term d is the average length of the indentation cavity inthe test diamond, as shown in FIG. 2 such that d=(d₁+d₂)/2. The term cis the average length of the radial cracks in the test diamond, as shownin FIG. 2 such the c=(c₁+c₂)/2.

Because of the uncertainties in determining hardness, identicalmeasurements were also performed on other diamonds. The measurements onother diamonds were found to be in agreement with published data on theother diamonds. The Vickers hardness tests were done on the (100) facesof the various types of diamonds in the (100) direction.

The indented surfaces of the annealed microwave plasma CVD-grownsingle-crystal diamonds as viewed by optical microscopy clearly differfrom those of other (softer) diamonds. The annealed microwave plasmaCVD-grown single-crystal diamond exhibits square crack patterns along<110> or <111>, no cross-like cracked lines along <100>, and awater-print-like deformation mark was produced on the surface of theannealed microwave plasma CVD-grown single-crystal diamond by thepyramidal Vickers indenter. In contrast, an annealed type IIa naturaldiamond has less square crack patterns along (110) and (111) but stillexhibits the cross-like (100) cracks of softer diamonds. Such resultsindicate that annealed microwave plasma CVD-grown single-crystal diamondis harder than the indenter, and the pressure due to plastic deformationof the indenter causes slippage of the softer {111} faces.

The Vickers indenters typically cracked after ˜15 measurements onunannealed microwave plasma CVD-grown single-crystal diamonds and typeIb natural diamonds. Further, The Vickers indenters typically crackedafter ˜5 measurements on annealed type IIa natural diamonds, annealedtype Ia natural diamonds and annealed type Ib HPHT synthetic diamonds.However, the Vickers indenter cracked after only one or two measurementson the annealed microwave plasma CVD-grown single-crystal diamonds.These observations further indicate that the annealed microwave plasmaCVD-grown single-crystal diamonds are harder than the measured valuesindicate. Indeed, many annealed microwave plasma CVD-grownsingle-crystal diamonds simply damaged the softer indenter. In suchinstances, the indenter left no imprint whatsoever in the surface of theannealed microwave plasma CVD-grown single-crystal diamonds.

FIG. 3 is a graph showing the hardness and toughness of annealedmicrowave plasma CVD-grown single-crystal diamonds in comparison to typeIIa natural diamonds annealed type IIa natural diamonds, annealed typeIa natural diamonds and annealed type Ib HPHT synthetic diamonds. Asshown in FIG. 3, the annealed microwave plasma CVD-grown single-crystaldiamonds have much higher hardness than type IIa natural diamond, asshown by the dotted square 10 in FIG. 3. All of the annealed microwaveplasma CVD-grown single-crystal diamonds also have a higher hardnessthan the reported range the reported range of hardness forpolycrystalline CVD diamonds, shown by the dotted square 20 in FIG. 3.The microwave plasma CVD-grown single-crystal diamonds represented inFIG. 3 have a fracture toughness of 6-10 MPa m^(1/2) with a hardness of140-180 GPa with indications that they may be harder.

As the present invention may be embodied in several forms withoutdeparting from the spirit or essential characteristics thereof, itshould also be understood that the above-described embodiments are notlimited by any of the details of the foregoing description, unlessotherwise specified, but rather should be construed broadly within itsspirit and scope as defined in the appended claims, and therefore allchanges and modifications that fall within the metes and bounds of theclaims, or equivalence of such metes and bounds are therefore intendedto be embraced by the appended claims.

1. A single crystal diamond grown by microwave plasma chemical vapordeposition annealed at pressures in excess of 4.0 GPa and heated totemperature in excess of 1500 degrees C. that has a hardness of greaterthan about 140 GPa, wherein the fracture toughness is >20 MPa m^(1/2).